Recently, from the standpoint of global conservation, it is a worldwide problem to suppress the total amount of carbon dioxide (CO2) exhausted from an internal combustion engine such as an automobile engine and the amount of nitrogen oxide (NOx) generated. In order to solve this problem, a lean-burn engine has been developed for the purpose of improving the fuel consumption and an absorption reduction-type NOx purifying catalyst, obtained by adding a function of absorbing NOx in a lean atmosphere to a conventional three-way catalyst, has been developed for the purpose of purifying the exhaust gas of the lean-burn engine. These are making certain progress in solving the above-described problems.
In the lean-burn engine combined with the absorption reduction-type NOx purifying catalyst, the fuel is usually burned at an air-fuel ratio in the lean (oxygen-excess) condition and temporarily burned in a stoichiometric (at a theoretical air-fuel ratio) or rich (fuel-excess) condition.
HC (hydrocarbon) or CO in the exhaust gas is efficiently burned and removed in the lean condition by the action of catalyst due to the oxidative atmosphere. On the other hand, NOx is captured by an absorbent in the lean condition and this is temporarily released in the stoichiometric or rich condition and reduced and purified by the action of a catalyst due to the reducing atmosphere.
By virtue of these combustion conditions and the action of the absorption reduction-type NOx purifying catalyst, as a whole, the fuel consumption is improved and at the same time, HC, CO and NOx in the exhaust gas can be purified with good efficiency.
In this absorption reduction-type NOx purifying catalyst, a noble metal such as platinum, gold, palladium and rhodium is used as the catalyst component and a basic substance such as alkali metal (e.g., potassium, sodium) and alkaline earth metal (e.g., calcium, barium) is used as the NOx absorbent.
This lean-burn system established by combining the control of an air-fuel ratio and the NOx absorbent is successful to a certain extent in solving the problem to improve the fuel consumption and reduce the total generation amount of CO, HC and NOx as compared with the conventional exhaust gas purifying system using a three-way catalyst and a nearly theoretical air-fuel ratio.
The techniques on this absorption reduction-type NOx purifying catalyst are described in Japanese Unexamined Patent Publication (Kokai) Nos. 7-51544, 7-136514, 9-24247 and 11-14422, filed by the present applicant, or the like.
In any absorption reduction-type NOx purifying catalyst of these prior techniques, an alkaline earth metal is used as the NOx absorbent and the catalyst component such as platinum and the NOx absorbent both are supported on a support such as γ-alumina.
However, these absorption reduction-type NOx purifying catalysts of conventional techniques have a problem that the NOx purifying capability is poor when the exhaust gas temperature is low and about 300° C. or less. In addition, SOx, of which source is sulfur contained in a slight amount, forms a salt with the NOx absorbent during the combustion and this SOx is not easily desorbed from the NOx absorbent. As a result, the NOx purifying capability disadvantageously decreases over time.
As such, conventional absorption reduction-type NOx purifying catalysts are in need of improvement in both the NOx purifying capability at low temperatures and the SOx desorbing property. Particularly, in order to apply the catalyst to a low-temperature exhaust gas containing a relatively large amount of SOx, such as diesel engine exhaust gas, those capabilities must be greatly improved.
Accordingly, an object of the present invention is to provide a catalyst having a different structure from conventional techniques and thereby provide an exhaust gas purifying catalyst freed from the above-described problems and capable of exhibiting a high NOx purifying capability at low temperatures and an improved SOx desorbing property.